There are numerous well known methods of encoding data for use in conjunction with a magnetic recording medium. One such method utilizes a binary ratio-encoding scheme involving pulse-width modulation of the digital information within a fixed time interval. In an article entitled "Ratio Recording for Lower Cassette Recording Costs" by Edgar Wolf, which appeared on page 76, of Computer Design for December 1972, the principles of ratio recording are succinctly described. By virtue of pulse width modulation, a binary logic ONE is indicated by a high level pulse having the width of 1/3 of a bit cell or bit time interval. A binary logic ZERO is represented by a pulse width of 2/3 of a bit interval. Since the pulse representing either a ONE or a ZERO has its leading edge occurring at the beginning of the bit time interval, a space of 2/3 or 1/3, respectively, of the bit time interval appears between the pulse and the next commencing bit cell.
By encoding the digital information using a ratio encoding scheme, it is possible to magnetize the recording medium in a first direction during the existence of the pulse, and magnetize the recording medium in a second direction for the space between the pulse and the end of the time interval. Utilizing ratio encoding schemes, a less expensive transport can then be used and reduced costs of recording and reading can be achieved. The data encoded by the ratio encoding scheme can be referred to as a self-clocking, serial data stream wherein the width of the pulse determines the binary ONE or ZERO and the time interval between the leading edge of a binary bit pulse in one cell and the leading edge of a binary bit pulse in the next consecutive cell defines the bit cell or time interval for the bit. This period t implies the clock rate 1/t. Thus, for example, if the bit cell interval is three milliseconds, the clock rate implied is 333 bits per second.
Generally, data coming from a computer or other source of information has its ONEs and ZEROs appearing as a constant level during a complete time interval or bit cell. The pulses are of equal widths and occur at a fixed clock time interval. This conventional data format must of necessity be generated in cooperation with a separate time base which causes each of the binary bits of information to be of equal duration. Such conventional data must be separated into a data signal appearing on one channel, and a separate external clock rate signal on another channel in order to be consistently recognizable, because the required timing information generally cannot be derived or reconstructed from the conventional data stream along. Thus, a minimum of two electrical signals are required for a conventional binary serial data transmission.
In the ratio encoding method, the separable parts of such a conventional data transmission are combined so that the data information as well as the external clock information can be transmitted coherently through a single binary serial channel. The hardware economy thus realized becomes apparent when the transmission medium is assumed, for example, to be a single-channel, binary, serial magnetic-tape recorder/reproducer system wherein blocks of digital data are recorded in conjunction with some physically arbitrarily real-time process and then are read out later in non-real time for subsequent analysis or processing. These two signals are synthesized by interlacing the external clock rate signal with the data in the form of pulse width modulated pulses. The two-channel information which is introduced in parallel to the ratio-encoding device is converted into a self-clocking, serial data stream on a single channel. The single-channel, ratio-encoded data can now be recorded onto a recording medium. Conversely, when reading information from a recording medium, the self-clocking, ratio-encoded information is formulated back into two separate channels, one of which includes the clock signal and the other of which includes the data in its conventional pulsed form having equal pulse widths for its ONE's and ZERO's.
In U.S. Pat. No. 2,887,679 entitled, "Pulse Width Memory Unit," issued May 19, 1959 to George B. Greene, a magnetic tape reader/recorder is described which uses the system of ratio encoding. In another U.S. Pat. No. 3,720,927 entitled "Speed Insensitive Reading and Writing Apparatus for Digital Information" issued Mar. 13, 1973 to Edgar Wolf, there is shown additional apparatus for reading and writing information onto a magnetic medium using ratio encoding techniques.
In each of the cases, it is necessary for the recorder/reproducer apparatus to combine a clock signal with the conventional data. Since the conventional data must of necessity have been generated in conjunction with a parent time base, it is necessary for the clock signal which is utilized in the ratio encoding scheme to be the same as the clock used as the time base for the conventional data stream.
In order to achieve this, it generally has been necessary that a recording system utilizing the ratio-encoding format originate the clock signal which synchronizes serial data introduced into the recorder. Or else, if an external clock is utilized, the frequency excursion of the clock, either intentional or caused by unavoidable environmental influences, must be substantially negligible and, in addition, the internal waveform generation process of the recorder must be accurately tuned to the incoming clock frequency in order to generate waveforms for recording which are subsequently, in the readout process, reliably distinguishable with regard to the binary sense of individual bits constructed as described above in self-clocking ratio format, just as it would be if the clock frequency were internally generated.
To accomplish this, previously described ratio-recording techniques have the serious drawback that the user of the peripheral magnetic recorder/reproducer must design his system so that the clock of the computer and the clock of the peripheral equipment correspond with each other. If the clock signals differ in their pulse rate, the reading or recording of the information will be impaired and the information will be distorted.